This invention relates to a process for pickling ferrous alloy steels (stainless steels). More particularly, this invention relates to a process for pickling hot rolled and annealed stainless steel strip using a pickling solution comprising hydrogen peroxide.
The annealing of stainless steel strip can result in the formation of oxides on the surface of the steel. These oxides, comprised of iron, chromium, nickel and other associated metal oxides, must be removed prior to utilizing the steel. However, the oxides of stainless steel are resistant to most of the common acid treatments. These oxides adhere tightly to the base metal, thus requiring mechanical scale cracking such as shot blasting, roll bending or leveling of the steel strip or electrolytic and/or molten salt bath treatment in order to either loosen these oxides or make the surface more porous prior to pickling. Traditionally, the oxides on the surface of the stainless steel have been removed, or xe2x80x9cpickled off,xe2x80x9d using nitric acid in combination with hydrofluoric acid.
There is a desire for a method of pickling stainless steels that eliminates the use of nitric acid.
The present invention relates to a process for pickling hot rolled, hot rolled and annealed, and cold rolled and annealed stainless steel strip in a continuous fashion. The process comprises immersing the stainless steel strip in a pre-pickling tank comprising a solution of sulfuric acid and hydrofluoric acid. The strip is then immersed in a pickling tank comprising a solution of sulfuric acid, hydrofluoric acid and hydrogen peroxide. In one embodiment, the pre-pickling tank (Tank 1 in FIG. 1) contains from about 90 g/l to about 200 g/l sulfuric acid and from about 10 g/l to about 60 g/l hydrofluoric acid. The solution in the pre-pickling tank is maintained at a temperature of from about 54xc2x0 C. to about 77xc2x0 C. The pickling tank (Tank 3 in FIG. 1) is comprised of a mixture of stabilized hydrogen peroxide, sulfuric acid, and hydrofluoric acid. In a specific embodiment, the pickling tank (Tank 3 in FIG. 1) is comprised of a mixture of stabilized hydrogen peroxide in a concentration of about 5 g/l to about 50 g/l, sulfuric acid in a concentration of about 20 g/l to about 60 g/l, and hydrofluoric acid in a concentration of about 2 g/l to about 50 g/l. In another embodiment, the concentration of stabilized hydrogen peroxide is from about 20 g/L to about 40 g/L. In a preferred embodiment, the concentration of stabilized hydrogen peroxide is from about 25 g/L to about 35 g/L. In another embodiment, the concentration of hydrogen peroxide is from about 5 g/L to less than 10 g/L. The solution in the pickling tank is maintained at a temperature of from about 20xc2x0 C. to about 60xc2x0 C. and is preferably at a temperature of from about 35 xc2x0 C. to about 50xc2x0 C.
Prior to immersing the steel strip in the pickling tank, the strip may be scrubbed, preferably using a scrubber-brush machine. In addition, the strip may also be immersed in a de-smutting tank immediately prior to being scrubbed. The de-smutting tank contains a solution comprising hydrogen peroxide, sulfuric acid and hydrofluoric acid, which is the overflow pickle solution form the pickling tank that is channeled back into the de-smutting tank.
In a separate embodiment, a filtration device and a heat exchanger are external to and coupled to the pickling tank. The filtration system device and heat exchanger are arranged in a re-circulating loop so that at any time, a portion of the solution from the pickling tank is routed through the filtration system device and heat exchanger. The resulting solution is deposited back into the pickling tank through at least one nozzle located inside the pickling tank.